The hydrocarbons, particularly the crude oil comprise compounds named asphaltenes that participate into the heavy fraction oils. These asphaltenes have variable compositions but they are almost all soluble in toluene. It is generally considered that the asphaltenes precipitate in the presence of an alkane, for example in presence of a hexane, heptane and pentane. The percentage of asphaltenes in crude oil is variable and can be up to 56% by weight of oil.
The oil fluids have the particularity to precipitate in temperature conditions and pressure that are difficult to predict.
The asphaltenes are responsible of the precipitation of the crude oil and of the clogging of wells or pipes. This precipitate is made under pressure variation accompanying the exit of the crude oil from the deposit up to the surface and during the transfer to the surface of the oil, even under pressure.
The knowledge of the temperature and the pressure, from which the precipitation of the asphaltene phase occurs, is important for optimizing oil extraction.
Also, the determination of the bubble pressure, i.e. the pressure, from which the first gas bubble appears under the effect of pressure decrease, when the oil passes from the liquid phase to the gas phase, it allows optimizing the oil extraction. In order to determine these parameters, there are standardized methods defined by the ASTN D.3279 and ASTM D.4124 rules.
The titration of the oil fluids, particularly the determination of the title in asphaltene, may prove to be essential, particularly for samples maintained in the deposit conditions (live oils) in relation to the samples for which these conditions were not maintained (dead oils). This titration can precisely allow the determination of the quantity of asphaltenes in risk of precipitating.
Otherwise, from the patent application FR-2 771 818, we know a method and a device allowing determining the thermodynamic characteristics of a polymer, for example the thermophysical properties and the phase transition envelope.
Nevertheless, with such a device, the measurements are made with a sample that was introduced in a device at a pressure that is not always representative of the origin pressure of the sample.
Furthermore, for a crude oil sample, if the pressure, at which the measurement is made, is less or equal to the bubble pressure, at least a partial precipitation of asphaltenes occurs. Thereby, the method described in the patent application FR-2 771 818 does not allow a generally reliable measurement.
Stachowak et al. (Transitiometric investigation of asphaltenic fluids under in-well pressure and temperature conditions, Energy and Fuels, vol. 15, September 2001, pages 1033-1037) describes a study on oil fluids in pressure and temperature conditions of deposit wells.
Nevertheless, the method described does not allow a precise measurement of the volume of the displaced mercury. The results of this method are distorted.
However, Bessières et al. (Design note: apparatus for simultaneous determination of the densities and heat capacities of liquids and of liquids with dissolved gas under an extended range of pressure (0.1-100 MPa, Measurement Science and Technology, vol. 11, May 1, 2000, pages N69-N72) describe the transfer of a fluid containing a certain quantity of gas at least from a pneumatic pump in order to carry out measurements by means of calorimetry.
Or, it is necessary to be able to carry out representative measurements of the crude oil sample behavior; both in the oil deposit and after the extraction of the sample and even after the variation of pressure and temperature that suffers the oil sample after its extraction.